Backoff based selection method of channels for data transmission

ABSTRACT

The invention relates to a method and device for sending data over a channel of a communication medium comprising a plurality of channels and which can be accessed by a plurality of nodes. Each node using a medium access mechanism with collision avoidance based on a computation of backoff values corresponding to a number of time-slots a node waits before accessing the communication medium. The method comprising the following steps performed by a first node of the plurality of nodes upon the reception of a trigger frame: obtaining a backoff counter value computed by the first node; selecting a channel among the plurality of channels based on the obtained backoff counter; and sending data on the selected channel by the first node.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Phase application of InternationalPatent Application No. PCT/EP2016/063397, filed Jun. 10, 2016, entitled“BACKOFF BASED SELECTION METHOD OF CHANNELS FOR DATA TRANSMISSION”,which claims priority to United Kingdom Patent Application No.1510363.3, filed Jun. 12, 2015 and United Kingdom Patent Application No.1515432.1, filed Aug. 1, 2015, all of which are hereby expresslyincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to communication networks andmore specifically to the sending of data over a channel of acommunication medium comprising a plurality of channels and which can beaccessed by a plurality of nodes.

BACKGROUND OF THE INVENTION

The IEEE 802.11 MAC standard family defines a way wireless local areanetworks (WLANs) can work at the physical and medium access control(MAC) level. Typically, the 802.11 MAC (Medium Access Control) operatingmode implements the well-known Distributed Coordination Function (DCF)which relies on a contention-based mechanism based on the so-called“Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA)technique.

The 802.11 medium access protocol standard or operating mode is mainlydirected to the management of communication nodes waiting for the mediumto become idle so as to try to access to the medium.

The network operating mode defined by the IEEE 802.11ac standardprovides very high throughput (VHT) by, among other means, moving fromthe 2.4 GHz band which is deemed to be highly susceptible tointerference to the 5 GHz band, thereby allowing for wider frequencycontiguous channels of 80 MHz, two of which may optionally be combinedto get a 160 MHz composite channel as operating band of the wirelessnetwork.

The 802.11ac standard also tweaks control frames such as theRequest-To-Send (RTS) and Clear-To-Send (CTS) frames to allow forcomposite channels of varying and predefined bandwidths of 20, 40 or 80MHz, the composite channels being made of one or more channels that arecontiguous within the operating band. The 160 MHz composite channel ispossible by the combination of two 80 MHz composite channels within the160 MHz operating band. The control frames specify the channel width(bandwidth) for the targeted composite channel.

A composite channel therefore consists of a primary channel on which agiven node performs EDCA backoff procedure to access the medium, and ofat least one secondary channel, of for example 20 MHz each. The primarychannel is used by the communication nodes to sense whether or not thechannel is idle, and the primary channel can be extended using thesecondary channel or channels to form a composite channel.

Given a tree breakdown of the operating band into elementary 20 MHzchannels, some secondary channels are named tertiary or quaternarychannels.

In 802.11ac, all transmissions, and thus all possible compositechannels, include a primary channel. This is because the nodes performfull Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) andNetwork Allocation Vector (NAV) tracking on the primary channel only.The other channels are assigned as secondary channels, on which thenodes have only capability of CCA (clear channel assessment), i.e.detection of an idle or busy state/status of said secondary channel.

An issue with the use of composite channels as defined in the 802.11n or802.11ac is that the 802.11n and 802.11ac-compliant nodes (i.e. HT nodesstanding for High Throughput nodes) and the other legacy nodes (i.e.non-HT nodes compliant only with for instance 802.11a/b/g) have toco-exist within the same wireless network and thus have to share the 20MHz channels.

To cope with this issue, the 802.11n and 802.11ac standards provide theability to duplicate control frames (e.g. RTS/CTS or CTS-to-Self) in an802.11a legacy format (called as “non-HT”) to establish a protection ofthe requested TXOP over the whole composite channel.

This is for any legacy 802.11a node that uses any of the 20 MHz channelinvolved in the composite channel to be aware of on-going communicationson the 20 MHz channel used. As a result, the legacy node is preventedfrom initiating a new transmission until the end of the currentcomposite channel TXOP granted to an 802.11n/ac node.

As originally proposed by 802.11n, a duplication of conventional 802.11aor “non-HT” transmission is provided to allow the two identical 20 MHznon-HT control frames to be sent simultaneously on both the primary andsecondary channels forming the targeted or requested composite channel.

This approach has been widened for 802.11ac to allow duplication overthe channels forming an 80 MHz or 160 MHz composite channel. In theremainder of the present document, the “duplicated non-HT frame” or“duplicated non-HT control frame” or “duplicated control frame” meansthat the node device duplicates the conventional or “non-HT”transmission of a given control frame over each secondary 20 MHz channelof the 40/80/160 MHz operating band.

In practice, to request a composite channel (equal to or greater than 40MHz) for a new TXOP, an 802.11n/ac node does an EDCA backoff procedurein the primary 20 MHz channel. In parallel, it performs a channelsensing mechanism, such as a Clear-Channel-Assessment (CCA) signaldetection, on the secondary channels to detect the secondary channel orchannels that are idle (channel state/status is “idle”) during a PIFSinterval before the start of the new TXOP (i.e. before the backoffcounter expires).

More recently, Institute of Electrical and Electronics Engineers (IEEE)officially approved the 802.11ax task group, as the successor of802.11ac. The primary goal of the 802.11ax task group consists inseeking for an improvement in data speed to wireless communicatingdevices used in dense deployment scenarios.

In such 802.11ax research context, the need becomes apparent forenhancing the efficiency and usage of the wireless channels. Typically,the user demands are primarily related to the delivery ofhigh-definition audio-visual real-time and interactive content in denseWLAN scenarios. It is well-known that the performance of the CSMA/CAprotocol used in the IEEE 802.11 standard deteriorates rapidly as thenumber of stations and the amount of traffic increase.

There is thus a need to improve the communication medium access formultiple users/stations for performing transmissions, and in particularconcurrent transmissions.

SUMMARY OF INVENTION

The present invention has been devised to overcome the foregoinglimitations.

This invention proposes one solution based on the reuse of the backoffcounter values for assigning a channel to a node of the network to senddata.

According to a first aspect, the invention relates to a method ofsending data over a channel of a communication medium comprising aplurality of channels and which can be accessed by a plurality of nodes,each node using a medium access mechanism with collision avoidance basedon a computation of backoff values corresponding to a number oftime-slots a node waits before accessing the communication medium. Themethod comprising the following steps performed by a first node of theplurality of nodes upon the reception of a trigger frame:

obtaining a backoff counter value computed by the first node;

selecting a channel among the plurality of channels based on theobtained backoff counter; and

sending data on the selected channel by the first node.

The channels are thus selected at low implementation cost.

In one embodiment, the method further comprises a step of determiningfrom the trigger frame the channels of the communication mediumavailable for contention by the plurality of nodes; wherein each channelis associated with a sequence number and wherein the selected channelhas an associated sequence number equal to the obtained backoff counter.

In a variant, the plurality of channels of the communication medium canbe accessed simultaneously by a plurality of nodes.

In one implementation, the plurality of channels are resource units ofan Orthogonal Frequency Division Multiple Access (OFDMA) scheme.

In another implementation, the plurality of channels are resource unitsof a Time Division Multiple Access (TDMA) scheme.

According to another aspect, the invention relates to a method ofsending data over a channel of a communication medium comprising aplurality of channels and which can be accessed by a plurality of nodes,each node using a medium access mechanism with collision avoidance basedon a computation of backoff values corresponding to a number oftime-slots a node waits before accessing the communication medium. Themethod comprising the following steps performed by a first node of theplurality of nodes upon the reception of a trigger frame:

determining from the trigger frame the number of channels of thecommunication medium available for contention by the plurality of nodes;

comparing the computed backoff value of the first node with the numberof available channels; and

if the computed backoff value is not greater than the number ofavailable channels, sending data over one of the channels of thecommunication medium.

In a preferred implementation, the available channels of thecommunication medium are sequentially numbered and wherein sending databy the first node is performed on the channel which number equals thecomputed backoff value.

In one implementation, the method further comprises a step ofdetermining if the first node is eligible to contend for access on achannel based on the backoff counter value and wherein the sending ofdata on the selected channel is performed if the first node isdetermined to be eligible.

In one implementation, the trigger frame reserves a transmissionopportunity on one or more channels the nodes can access using acontention scheme.

In one implementation, the channel is a resource unit resulting from thedivision of a 20 MHz channel into OFDMA sub-channels.

In one implementation, the trigger frame is received from the accesspoint and wherein the data is transmitted at the sending step to theaccess point.

In one implementation, the method further comprises a step of indexingthe plurality of channels and wherein the selecting of a channel fordata transmission is derived from a mapping between backoff countervalues and indexes of the plurality of channels.

According to a variant of the first aspect, the method relates to awireless communication method in a wireless network comprising an accesspoint and a plurality of nodes, the method comprising the followingsteps, at one of said nodes:

receiving a trigger frame, the trigger frame reserving at least onetransmission opportunity on one or more random channels the nodes canaccess using a contention scheme;

obtaining a random backoff value local to the node;

selecting a random channel based on the obtained backoff counter; and

transmitting data on the selected random channel during a reservedtransmission opportunity.

According to another aspect, the invention relates to a communicationdevice in a wireless network comprising an access point and a pluralityof nodes, the communication device being one of the nodes andcomprising:

means for receiving a trigger frame, the trigger frame reserving atleast one transmission opportunity on one or more random channels thenodes can access using a contention scheme;

means for obtaining a backoff counter value computed by the node;

means for selecting a random channel among the plurality of randomchannels based on the obtained backoff counter; and sending data on theselected channel by the node.

According to another aspect, the invention relates to a wirelesscommunication method in a wireless network comprising an access pointand a plurality of nodes, the method comprising the following steps, atone of said nodes:

receiving a trigger frame, the trigger frame reserving a transmissionopportunity on one or more random channels the nodes can access;

obtaining a random backoff value local to the node;

comparing the backoff value to a threshold value;

determining if the node is eligible to contend for access on a randomchannel based on the result of the comparing; and

transmitting data on a random channel during the transmissionopportunity if the node is determined to be eligible.

This advantageously limits the number of stations allowed to transmit ona random channel during the transmission opportunity: as a consequence,this reduces the collision probability for random onto a random channel.

In one implementation, the node is eligible to contend for access if therandom backoff value is not greater than the threshold value.

In one implementation, the threshold value is based on the number ofrandom channels the nodes can access using a contention scheme.

In one implementation, the threshold value is received from the accesspoint. This advantageously offers a means for access point to controlthe contention over the random channels, by limiting/increasing thenumber of stations allowed to try to transmit during the transmissionopportunity.

In one implementation, the threshold value is included in the triggerframe. In one implementation, the node comprises a plurality oftransmission queues for serving data at different priorities.

In one implementation, the steps of obtaining a random backoff value andcomparing the backoff value to a threshold value are executed for eachtransmission queue,

and wherein a queue is eligible to serve data depending on the result ofcomparing the random backoff value associated with said queue with thethreshold value.

In one implementation, a queue is eligible to serve data if the randombackoff value associated with said queue is not greater than thethreshold value.

In one implementation, the node is eligible to contend for access if atleast one of its queues is eligible to serve data.

In one variant, if a plurality of queues are eligible to serve data, onequeue is selected among the plurality of eligible queues for servingdata to be transmitted on the random channel in the transmitting step.

In another variant, if a plurality of queues are eligible to serve dataand the node is capable of transmitting data over a plurality oftransmission opportunities on one or more random channels, a pluralityof eligible queues are selected for serving data to be transmitted onone or more random channels in the transmitting step.

In one implementation of one of the two variants, the selection of oneor a plurality of eligible queue(s) is based on the traffic priorityassociated with said queue or queues or on the random backoff value(s)obtained for said queue or queues.

In one implementation, the random channel on which data is transmittedis selected among a plurality of random channels the nodes can accessusing a contention scheme based on the obtained random backoff value.

In one implementation, the random channel is a resource unit resultingfrom the division of a 20 MHz channel into OFDMA sub-channels.

In one implementation, the trigger frame is received from the accesspoint and wherein the data is transmitted at the transmitting step tothe access point.

In one implementation, the random backoff value is the number of 802.11time-slots the node must wait when a medium is sensed idle before thenode grants itself a transmission opportunity (TXOP).

In one implementation, the method further comprising a step of updatinga random backoff value by subtracting the number of random channels tobe used for a next data transmission.

According to another aspect, the invention relates to a communicationdevice in a wireless network comprising an access point and a pluralityof nodes, the communication device being one of the nodes andcomprising:

means for receiving a trigger frame, the trigger frame reserving atransmission opportunity on a random channel the nodes can access usinga contention scheme;

means for obtaining a random backoff value local to the node;

means for comparing the backoff value to a threshold value;

means for determining if the node is eligible to contend for access onthe random channel based on the result of the comparing; and

means for transmitting data on the random channel during thetransmission opportunity if the node is determined to be eligible.

The invention also relates to a wireless communication system having anaccess point and at least one node according to one of the aboveaspects.

The invention also relates in its other aspects to an apparatus and acomputer program implementing the steps of the method and anon-transitory computer-readable medium storing such a program.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will become apparent tothose skilled in the art upon examination of the drawings and detaileddescription. Embodiments of the invention will now be described, by wayof example only, and with reference to the following drawings.

FIG. 1 illustrates a typical wireless communication system in whichembodiments of the invention may be implemented.

FIG. 2 is a timeline schematically illustrating a conventionalcommunication mechanism according to the IEEE 802.11 standard.

FIGS. 3a, 3b and 3c illustrate the IEEE 802.11e EDCA involving accesscategories.

FIG. 4 illustrates 802.11ac channel allocation that supports compositechannel bandwidths of 20 MHz, 40 MHz, 80 MHz or 160 MHz.

FIGS. 5a and 5b illustrate different examples of uplink OFDMAtransmission scheme, wherein the AP issues a Trigger Frame for reservinga transmission opportunity of OFDMA sub-channels (resource units) on an80 MHz channel.

FIG. 6 shows a schematic representation a communication device orstation in accordance with embodiments of the present invention.

FIG. 7 shows a block diagram schematically illustrating the architectureof a wireless communication device in accordance with embodiments of thepresent invention.

FIGS. 8a and 8b illustrate, using two flowcharts, general steps of afirst exemplary embodiment of the present invention to provide randomaccess to OFDMA Resource Unit for (uplink) multi-user OFDMAtransmission.

FIG. 9 and FIG. 13 illustrate exemplary communication lines resultingfrom an implementation of the embodiments of FIGS. 8a and 8 b.

FIGS. 10 and 11 depicts the format of a ‘RU Information Element’, whichmay be used according to embodiments of the present invention.

FIG. 12 illustrates the behaviour of a source node 600 for contendingaccess to a communication channel according to another exemplaryembodiment of the invention.

FIG. 14 illustrates, using a flowchart, a possible implementation of avirtual collision handler.

DETAILED DESCRIPTION

The invention will now be described by means of specific non-limitingexemplary embodiments and by reference to the figures.

FIG. 1 illustrates a communication system in which several communicationnodes or stations 101-107 (STA₁-STA₇) exchange data frames over a radiotransmission channel 100 of a wireless local area network (WLAN), underthe management of a central station or access point (AP) 110. The radiotransmission channel 100 is defined by an operating frequency bandconstituted typically by a single 20 MHz channel or a plurality of 20MHz channels forming a composite channel.

Access to the shared radio medium to send data frames may be based onthe CSMA/CA technique, for sensing the carrier and avoiding collisionsby separating concurrent transmissions in space and time.

Carrier sensing in CSMA/CA is performed by both physical and virtualmechanisms. Virtual carrier sensing is achieved by transmitting controlframes to reserve the medium prior to transmission of data frames.

Next, a source node first attempts through the physical mechanism, tosense a medium that has been idle for at least one DIFS (standing forDCF InterFrame Spacing) time period, before transmitting data frames.

However, if it is sensed that the shared radio medium is busy during theDIFS period, the source node continues to wait until the radio mediumbecomes idle. To do so, it starts a countdown backoff counter designedto expire after a number of timeslots chosen randomly between [0, CW],CW (integer) being referred to as the Contention Window. This backoffmechanism or procedure is the basis of the collision avoidance mechanismthat defers the transmission time for a random interval, thus reducingthe probability of collisions on the shared channel. After the randominterval, the source node may send data or control frames if the mediumis idle.

One problem of wireless data communications is that it is not possiblefor the source node to listen while sending, thus preventing the sourcenode from detecting data corruption due to channel fading orinterference or collision phenomena. A source node remains unaware ofthe corruption of the data frames sent and continues to transmit theframes unnecessarily, thus wasting access time.

The Collision Avoidance mechanism of CSMA/CA thus provides positiveacknowledgement (ACK) of the sent data frames by the receiving node ifthe frames are received with success, to notify the source node that nocorruption of the sent data frames occurred.

The ACK is transmitted at the end of reception of the data frame,immediately after a period of time called Short InterFrame Space (SIFS).

If the source node does not receive the ACK within a specified ACKtimeout or detects the transmission of a different frame on the channel,it may infer data frame loss. In that case, it generally reschedules theframe transmission according to the above-mentioned backoff procedure.However, this can be seen as a bandwidth waste if only the ACK has beencorrupted but the data frames were correctly received by the receivingnode.

To improve the Collision Avoidance efficiency of CSMA/CA, a four-wayhandshaking mechanism is optionally implemented. One implementation isknown as the RTS/CTS exchange, defined in the 802.11 standard.

The RTS/CTS exchange consists in exchanging control frames to reservethe radio medium prior to transmitting data frames during a transmissionopportunity called TXOP in the 802.11 standard as described below, thusprotecting data transmissions from any further collisions.

FIG. 2 illustrates the behaviour of three groups of nodes during aconventional communication over a 20 MHz channel of the 802.11 medium:transmitting or source node 20, receiving or addressee or destinationnode 21 and other nodes 22 not involved in the current communication.

Upon starting the backoff process 270 prior to transmitting data, astation e.g. source node 20, initializes its backoff time counter to arandom value as explained above. The backoff time counter is decrementedonce every time slot interval 260 for as long as the radio medium issensed idle (countdown starts from T0, 23 as shown in the Figure).

The time unit in the 802.11 standard is the slot time called ‘aSlotTime’parameter. This parameter is specified by the PHY (physical) layer (forexample, aSlotTime is equal to 9 μs for the 802.11n standard). Alldedicated space durations (e.g. backoff) add multiples of this time unitto the SIFS value.

The backoff time counter is ‘frozen’ or suspended when a transmission isdetected on the radio medium channel (countdown is stopped at T1, 24 forother nodes 22 having their backoff time counter decremented).

The countdown of the backoff time counter is resumed or reactivated whenthe radio medium is sensed idle anew, after a DIFS time period. This isthe case for the other nodes at T2, 25 as soon as the transmissionopportunity TXOP granted to source node 20 ends and the DIFS period 28elapses. DIFS 28 (DCF inter-frame space) thus defines the minimumwaiting time for a source node before trying to transmit some data. Inpractice, DIFS=SIFS+2*aSlotTime.

When the backoff time counter reaches zero (26) at T1, the timerexpires, the corresponding node 20 requests access onto the medium inorder to be granted a TXOP, and the backoff time counter isreinitialized 29 using a new random backoff value.

In the example of the Figure implementing the RTS/CTS scheme, at T1, thesource node 20 that wants to transmit data frames 230 sends a specialshort frame or message acting as a medium access request to reserve theradio medium, instead of the data frames themselves, just after thechannel has been sensed idle for a DIFS or after the backoff period asexplained above.

The medium access request is known as a Request-To-Send (RTS) message orframe. The RTS frame generally includes the addresses of the source andreceiving nodes (“destination 21”) and the duration for which the radiomedium is to be reserved for transmitting the control frames (RTS/CTS)and the data frames 230.

Upon receiving the RTS frame and if the radio medium is sensed as beingidle, the receiving node 21 responds, after a SIFS time period 27 (forexample, SIFS is equal to 16 μs for the 802.11n standard), with a mediumaccess response, known as a Clear-To-Send (CTS) frame. The CTS framealso includes the addresses of the source and receiving nodes, andindicates the remaining time required for transmitting the data frames,computed from the time point at which the CTS frame starts to be sent.

The CTS frame is considered by the source node 20 as an acknowledgmentof its request to reserve the shared radio medium for a given timeduration.

Thus, the source node 20 expects to receive a CTS frame 220 from thereceiving node 21 before sending data 230 using unique and unicast (onesource address and one addressee or destination address) frames.

The source node 20 is thus allowed to send the data frames 230 uponcorrectly receiving the CTS frame 220 and after a new SIFS time period27.

An ACK frame 240 is sent by the receiving node 21 after having correctlyreceived the data frames sent, after a new SIFS time period 27.

If the source node 20 does not receive the ACK 240 within a specifiedACK Timeout (generally within the TXOP), or if it detects thetransmission of a different frame on the radio medium, it reschedulesthe frame transmission using the backoff procedure anew.

Since the RTS/CTS four-way handshaking mechanism 210/220 is optional inthe 802.11 standard, it is possible for the source node 20 to send dataframes 230 immediately upon its backoff time counter reaching zero (i.e.at T1).

The requested time duration for transmission defined in the RTS and CTSframes defines the length of the granted transmission opportunity TXOP,and can be read by any listening node (“other nodes 22” in FIG. 2) inthe radio network.

To do so, each node has in memory a data structure known as the networkallocation vector or NAV to store the time duration for which it isknown that the medium will remain busy. When listening to a controlframe (RTS 210 or CTS 220) not addressed to itself, a listening node 22updates its NAVs (NAV 255 associated with RTS and NAV 250 associatedwith CTS) with the requested transmission time duration specified in thecontrol frame. The listening nodes 22 thus keep in memory the timeduration for which the radio medium will remain busy.

Access to the radio medium for the other nodes 22 is consequentlydeferred 30 by suspending 31 their associated timer and then by laterresuming 32 the timer when the NAV has expired.

This prevents the listening nodes 22 from transmitting any data orcontrol frames during that period.

It is possible that receiving node 21 does not receive RTS frame 210correctly due to a message/frame collision or to fading. Even if it doesreceive it, receiving node 21 may not always respond with a CTS 220because, for example, its NAV is set (i.e. another node has alreadyreserved the medium). In any case, the source node 20 enters into a newbackoff procedure.

The RTS/CTS four-way handshaking mechanism is very efficient in terms ofsystem performance, in particular with regard to large frames since itreduces the length of the messages involved in the contention process.

In detail, assuming perfect channel sensing by each communication node,collision may only occur when two (or more) frames are transmittedwithin the same time slot after a DIFS 28 (DCF inter-frame space) orwhen their own backoff counter has reached zero nearly at the same timeT1. If both source nodes use the RTS/CTS mechanism, this collision canonly occur for the RTS frames. Fortunately, such collision is earlydetected by the source nodes since it is quickly determined that no CTSresponse has been received.

FIGS. 3a, 3b and 3c illustrate the IEEE 802.11e EDCA involving accesscategories, in order to improve the quality of service (QoS). In theoriginal standard, a communication node includes only one transmissionqueue/buffer. However, since a subsequent data frame cannot betransmitted until the transmission/retransmission of a preceding frameends, the delay in transmitting/retransmitting the preceding frameprevents the communication from having QoS.

The IEEE 802.11e has overturned this deficiency in providing quality ofservice (QoS) enhancements to make more efficient use of the wirelessmedium.

This standard relies on a coordination function, called hybridcoordination function (HCF), which has two modes of operation: enhanceddistributed channel access (EDCA) and HCF controlled channel access(HCCA).

EDCA enhances or extends functionality of the original access DCFmethod: EDCA has been designed for support of prioritized trafficsimilar to DiffSery (Differentiated Services), which is a protocol forspecifying and controlling network traffic by class so that certaintypes of traffic get precedence.

EDCA is the dominant channel access mechanism in WLANs because itfeatures a distributed and easily deployed mechanism.

The above deficiency of failing to have satisfactory QoS due to delay inframe retransmission can be solved with a plurality of transmissionqueues/buffers.

QoS support in EDCA is achieved with the introduction of four AccessCategories (ACs), and thereby of four corresponding transmission queuesor buffers (310).

Each AC has its own transmission queue/buffer to store correspondingdata frames to be transmitted on the network. The data frames, namelythe MSDUs, incoming from an upper layer of the protocol stack are mappedonto one of the four AC queues/buffers and thus input in the mapped ACbuffer.

Each AC has also its own set of channel access parameters, and isassociated with a priority value, thus defining traffic of higher orlower priority of MSDUs.

That means that each AC (and corresponding buffer) acts as anindependent DCF contending entity including its respective backoffengine 311. In other words, the ACs within the same communication nodecompete one with each other to access the wireless medium and to obtaina transmission opportunity, using the contention mechanism as explainedabove with reference to FIG. 2 for example.

Service differentiation between the ACs is achieved by setting differentcontention window parameters (CWmin, CWmax), arbitrary interframe spaces(AIFS), and transmission opportunity duration limits (TXOP_Limit).

With EDCA, high priority traffic has a higher chance of being sent thanlow priority traffic: a node with high priority traffic waits a littleless (low CW) before it sends its packet, on average, than a node withlow priority traffic.

The four AC buffers (310) are shown in FIG. 3 a.

Buffers AC3 and AC2 are usually reserved for real-time applications(e.g., voice or video transmission). They have, respectively, thehighest priority and the last-but-one highest priority.

Buffers AC1 and AC0 are reserved for best effort and background traffic.They have, respectively, the last-but-one lowest priority and the lowestpriority.

Each data unit, MSDU, arriving at the MAC layer from an upper layer(e.g. Link layer) with a priority is mapped into an AC according tomapping rules. FIG. 3b shows an example of mapping between eightpriorities of traffic class (User Priorities or UP, 0-7 according IEEE802.1d) and the four ACs. The data frame is then stored in the buffercorresponding to the mapped AC.

When the backoff procedure of an AC ends, the MAC controller (reference704 in FIG. 7 below) of the transmitting node transmits a data framefrom this AC to the physical layer for transmission onto the wirelesscommunication network.

Since the ACs operate concurrently in accessing the wireless medium, itmay happen that two ACs of the same communication node have theirbackoff ending simultaneously. In such a situation, a virtual collisionhandler (312) of the MAC controller operates a selection of the AChaving the highest priority between the conflicting ACs, and gives uptransmission of data frames from the ACs having lower priorities.

Then, the virtual collision handler commands those ACs having lowerpriorities to start again a backoff operation using an increased CWvalue.

FIG. 3c illustrates configurations of a MAC data frame and a QoS controlfield (300) included in the header of the IEEE 802.11e MAC frame.

The MAC data frame also includes, among other fields, a Frame Controlheader (301) and a frame body (302).

As represented in the Figure, the QoS control field 300 is made of twobytes, including the following information items:

-   -   Bits B0 to B3 are used to store a traffic identifier (TID) which        identifies a traffic stream. The traffic identifier takes the        value of the transmission priority value (User Priority UP,        value between 0 and 7—see FIG. 3b ) corresponding to the data        conveyed by the data frame or takes the value of a traffic        stream identifier (TSID, value between 8 and 15) for other data        streams.    -   Bit B4 is set to 1 and is not detailed here.    -   Bits B5 and B6 define the Ack policy subfield which specifies        the acknowledgment policy associated with the data frame. This        subfield is used to determine how the data frame has to be        acknowledged by the receiving node. Usually, it may take three        different values as follows:        -   value equal to “Normal ACK” in the case where the            transmitting node or source node requires a conventional            acknowledgment to be sent (by the receiving node) after a            short interframe space (SIFS) period following the            transmission of the data frame;        -   value equal to “No ACK” in the case where the source node            does not require acknowledgment. That means that the            receiving node takes no action upon receipt of the data            frame; and        -   value equal to “Block ACK” for acknowledgment per block of            MSDUs. The Block Ack scheme allows two or more data frames            230 to be transmitted before a Block ACK frame is returned            to acknowledge the receipt of the data frames. The Block ACK            increases communication efficiency since only one signalling            ACK frame is needed to acknowledge a block of frames, while            every ACK frame originally used has a significant overhead            for radio synchronization. The receiving node takes no            action immediately upon receiving the last data frame,            except the action of recording the state of reception in its            scoreboard context. With such a value, the source node is            expected to send a Block ACK request (BAR) frame, to which            the receiving node responds using the procedure described            below.    -   Bit B7 (350) is reserved (not used by the current 802.11        standards).    -   Bits B8-B15 indicate the amount of buffered traffic for a given        TID at the non-AP station sending this frame. The AP may use        this information to determine the next TXOP duration it will        grant to the station. A queue size of 0 indicates the absence of        any buffered traffic for that TID.

To meet the ever-increasing demand for faster wireless networks tosupport bandwidth-intensive applications, 802.11ac is targeting largerbandwidth transmission through multi-channel allocations. FIG. 4illustrates 802.11ac channel allocation that supports composite channelbandwidths of 20 MHz, 40 MHz, 80 MHz or 160 MHz.

IEEE 802.11ac introduces support of a restricted number of predefinedsubsets of 20 MHz channels to form the sole predefined compositechannels configurations that are available for reservation by any802.11ac node on the wireless network to transmit data.

The predefined subsets are shown in the figure and correspond to 20 MHz,40 MHz, 80 MHz, and 160 MHz composite channel bandwidths, compared toonly 20 MHz and 40 MHz supported by 802.11n. Indeed, the 20 MHzcomponent channels 400-1 to 400-8 are concatenated to form widercommunication composite channels.

In the 802.11ac standard, the channels of each predefined 40 MHz, 80 MHzor 160 MHz subset are contiguous within the operating band, i.e. no hole(missing channel) in the composite channel as ordered in the operatingband is allowed.

The 160 MHz channel bandwidth is composed of two 80 MHz channels thatmay or may not be frequency contiguous (while being contiguous withinthe operating band). The 80 MHz and 40 MHz channels are respectivelycomposed of two frequency adjacent or contiguous 40 MHz and 20 MHzchannels, respectively.

Note that embodiments of the invention apply to any composition ofchannel bandwidth, including any contiguous and non contiguouspredefined subset within the operating band.

A node is granted a TXOP through the enhanced distributed channel access(EDCA) mechanism on the “primary channel” (400-3). Indeed, for eachcomposite channel, 802.11ac designates one 20 MHz channel as “primary”,meaning that it is used for contending for access to the compositechannel. The primary 20 MHz channel is common to all nodes (STAs)belonging to the same basic set, i.e. managed by or registered to thesame local Access Point (AP).

However, to make sure that no other legacy nodes (i.e. nodes havingtheir primary channel mapped to a secondary channel of the basic set)uses the secondary channels, it is provided that the control frames(e.g. RTS frame/CTS frame) reserving the composite channel areduplicated over each 20 MHz channel of such composite channel.

As addressed earlier, the IEEE 802.11ac standard enables up to four, oreven eight, 20 MHz channels to be bound. Because of the limited numberof channels (19 in the 5 GHz band in Europe), channel saturation becomesproblematic. Indeed, in densely populated areas, the 5 GHz band willsurely tend to saturate even with a 20 or 40 MHz bandwidth usage perWireless-LAN cell.

Developments in the 802.11ax standard seek to enhance efficiency andusage of the wireless channel for dense environments.

In this perspective, one may consider multi-user (MU) transmissionfeatures, allowing multiple simultaneous transmissions to/from differentusers in both downlink and uplink directions. In the uplink (UL),multi-user transmissions can be used to mitigate the collisionprobability by allowing multiple nodes to simultaneously transmit.

To actually perform such multi-user transmission, it has been proposedto split a granted 20 MHz channel into sub-channels (elementarysub-channels), also referred to as resource units (RUs), that are sharedin the frequency domain by multiple users, based for instance onOrthogonal Frequency Division Multiple Access (OFDMA) technique.

OFDMA is a multi-user variation of OFDM which has emerged as a new keytechnology to improve efficiency in advanced infrastructure-basedwireless networks. It combines OFDM on the physical layer with FrequencyDivision Multiple Access (FDMA) on the MAC layer, allowing differentsubcarriers to be assigned to different nodes in order to increaseconcurrency. Adjacent sub-carriers often experience similar channelconditions and are thus grouped to sub-channels: an OFDMA sub-channel orRU is thus a set of sub-carriers.

As currently envisaged for 802.11ax, the granularity of such OFDMAsub-channels is finer than the original 20 MHz channel band. Typically,a 2 MHz or 5 MHz OFDMA sub-channel may be contemplated as a minimalwidth, therefore defining for instance 9 OFDMA sub-channels or resourceunits within a single 20 MHz channel.

FIG. 5a illustrates an example of uplink OFDMA transmission scheme. Inthis example, each 20 MHz channel (400-1, 400-2, 400-3 or 400-4) issub-divided in frequency domain in four OFDMA sub-channels or RUs 410 ofsize 5 MHz. These sub-channels (or resource units or sets of“sub-carriers”) may also be referred to as “traffic channels”.

Of course the number of RUs splitting a 20 MHz channel may be differentfrom four. For instance, between two to nine RUs may be provided (thuseach having a size between 10 MHz and about 2 MHz).

To support multi-user uplink, e.g. uplink transmission to the 802.11axaccess point (AP) during the granted TXOP, the 802.11ax AP has toprovide signalling information for the legacy nodes (non-802.11ax nodes)to set their NAV and for the 802.11ax nodes to determine the allocationof the resource units RUs.

In order to trigger uplink communications from the nodes, a triggerframe (TF) 430 may be sent by the AP. The trigger frame reserves atransmission opportunity on one or more channels of the wireless networkfor the plurality of nodes.

The trigger frame may designate the resource units (RUs) 410 that can beaccessed by the stations for their uplink communications. Designated RUsmay be “Random RUs” dedicated for contention based random access. Inother words, Random RUs designated or allocated by the AP in the TF mayserve as basis for contention between stations willing to access thecommunication medium for sending data frames. A collision occurs whentwo or more stations attempt to transmit at the same time over the sameRU. Designated RUs may also be “scheduled RUs”, in addition or inreplacement of the Random RUs. Scheduled RUs may be reserved for certainstations, in which case no contention for accessing such RUs is neededfor sending data frames.

Alternatively, a trigger frame may designate only RUs of random type(Random RUs). This trigger frame is referred to as a trigger frame forrandom access (TF-R). A TF-R may be emitted by the AP to allow multiplestations to perform UL MU (UpLink Multi-User) random access to acquire aTXOP for their UL transmissions.

The trigger frame thus defines according to one implementation resourceunits including a plurality of random resource units that the nodes canaccess using a contention scheme.

The trigger frame may include additional signalling towards thestations, such as the bandwidth or width of the targeted compositechannel, meaning that a 20, 40, 80 or 160 MHz value is coded in thetrigger frame.

The TF frame may be sent over the primary 20 MHz channel and replicated(duplicated) on each other 20 MHz channel forming the bandwidth of thetargeted composite channel. As described above for the duplication ofcontrol frames, it is expected that every nearby legacy node (non-HT or802.11ac node) receiving the TF on its primary channel sets its NAV tothe value specified in the TF frame. This prevents these legacy nodesfrom accessing the channels of the targeted composite channel during theTXOP.

Embodiments of the invention rely advantageously on the division of 20MHz channels into sub-channels (RUs), which allows for optimizing theusage of the bandwidth by the multiple stations of the network.

The sub-division of the 20 MHz channels can be performed in thefrequency domain and shared by multiple users using for example theOrthogonal Frequency Division Multiple Access (OFDMA) technique asdiscussed above. This results into OFDMA sub-channels.

The sub-division can also be performed in the time domain relying onTDMA (Time Division Multiple Access). This results into TDMAsub-channels. FIG. 5b illustrates an example of uplink TDMA transmissionscheme. In this example, the TXOP is sub-divided in time domain in TDMAsub-channels 420 of 20 MHz width. Any combination of TDMA and OFDMA canalso be envisaged for defining the RUs.

More generally, embodiments of the invention may rely on a plurality ofchannels which can be accessed by a plurality of nodes (multi-userchannels), wherein a channel may correspond to:

-   -   a RU (e.g. OFDMA or TDMA sub-channel) resulting from the        sub-division of a 20 MHz channel as described above;    -   a 20 MHz channel itself, the plurality of channels may thus        correspond to the composite channel or part of the composite        channel (or inversely, a channel results from the division of        the composite channel or part of the composite channel); or    -   any communication resource of the communication medium        comprising a plurality of resources that can be accessed by a        plurality of nodes.

A trigger frame in this context defines communication channels includinga plurality of random channels that the nodes access using a contentionscheme.

Embodiments of the invention provide a contention procedure to be usedby the nodes to randomly access the plurality of channels and transmitdata, i.e. the embodiments define a multi-user channels access andtransmission method.

More particularly and in order to improve the efficiency of the systemwith regards to un-managed traffic towards the AP (for example, uplinkmanagement frames from associated stations, unassociated nodes intendingto reach an AP, or simply un-managed data traffic), there is a need forrandom access onto the multi-user TXOP over the available RUs discussedabove. In this context, there is a need for a random procedure forselecting the stations allowed to transmit in the multi-user TXOP and/orselecting the OFDMA sub-channel(s) to be accessed to by the selectedstations.

FIG. 6 schematically illustrates a communication device 600 of the radionetwork 100, configured to implement at least one embodiment of thepresent invention. The communication device 600 may preferably be adevice such as a microcomputer, a workstation or a light portabledevice. The communication device 600 comprises a communication bus 613to which there are preferably connected:

-   -   a central processing unit 611, such as a microprocessor, denoted        CPU;    -   a read only memory 607, denoted ROM, for storing computer        programs for implementing the invention;    -   a random access memory 612, denoted RAM, for storing the        executable code of methods according to embodiments of the        invention as well as the registers adapted to record variables        and parameters necessary for implementing methods according to        embodiments of the invention; and    -   at least one communication interface 602 connected to the radio        communication network 100 over which digital data packets or        frames or control frames are transmitted, for example a wireless        communication network according to the 802.11ax protocol. The        frames are written from a FIFO sending memory in RAM 612 to the        network interface for transmission or are read from the network        interface for reception and writing into a FIFO receiving memory        in RAM 612 under the control of a software application running        in the CPU 611.

Optionally, the communication device 600 may also include the followingcomponents:

-   -   a data storage means 604 such as a hard disk, for storing        computer programs for implementing methods according to one or        more embodiments of the invention;    -   a disk drive 605 for a disk 606, the disk drive being adapted to        read data from the disk 606 or to write data onto said disk;    -   a screen 609 for displaying decoded data and/or serving as a        graphical interface with the user, by means of a keyboard 610 or        any other pointing means.

The communication device 600 may be optionally connected to variousperipherals, such as for example a digital camera 608, each beingconnected to an input/output card (not shown) so as to supply data tothe communication device 600.

Preferably the communication bus provides communication andinteroperability between the various elements included in thecommunication device 600 or connected to it. The representation of thebus is not limiting and in particular the central processing unit isoperable to communicate instructions to any element of the communicationdevice 600 directly or by means of another element of the communicationdevice 600.

The disk 606 may optionally be replaced by any information medium suchas for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, aUSB key or a memory card and, in general terms, by an informationstorage means that can be read by a microcomputer or by amicroprocessor, integrated or not into the apparatus, possibly removableand adapted to store one or more programs whose execution enables amethod according to the invention to be implemented.

The executable code may optionally be stored either in read only memory607, on the hard disk 604 or on a removable digital medium such as forexample a disk 606 as described previously. According to an optionalvariant, the executable code of the programs can be received by means ofthe communication network 603, via the interface 602, in order to bestored in one of the storage means of the communication device 600, suchas the hard disk 604, before being executed.

The central processing unit 611 is preferably adapted to control anddirect the execution of the instructions or portions of software code ofthe program or programs according to the invention, which instructionsare stored in one of the aforementioned storage means. On powering up,the program or programs that are stored in a non-volatile memory, forexample on the hard disk 604 or in the read only memory 607, aretransferred into the random access memory 612, which then contains theexecutable code of the program or programs, as well as registers forstoring the variables and parameters necessary for implementing theinvention.

In a preferred embodiment, the apparatus is a programmable apparatuswhich uses software to implement the invention. However, alternatively,the present invention may be implemented in hardware (for example, inthe form of an Application Specific Integrated Circuit or ASIC).

FIG. 7 is a block diagram schematically illustrating the architecture ofthe communication device 600 in its role as one of the nodes (orstations) 101-107 or an AP 110 according to embodiments of theinvention. As illustrated, node 600 comprises a physical (PHY) layerblock 703, a MAC layer block 702, and an application layer block 701.

The PHY layer block 703 (e.g. a 802.11 standardized PHY layer) has thetask of formatting, modulating on or demodulating from any channel, andsending or receiving frames over the radio medium (transmission channel)100 used. The transmitted/received frames may be 802.11 frames; forinstance medium access trigger frames of the TF 430 type to reserve atransmission slot, MAC data and management frames base on a 20 MHz widthto interact with legacy 802.11 stations, as well as of MAC data framesof OFDMA type having smaller width than 20 MHz legacy (typically 2 or 5MHz) to/from that radio medium.

The MAC layer block or controller 702 preferably comprises a MAC 802.11layer 704 implementing conventional 802.11ax MAC operations, and anadditional block 705 for carrying out, at least partially, embodimentsof the invention. The MAC layer block 702 may optionally be implementedin software, which software is loaded into RAM 612 and executed by CPU611.

Preferably, the additional block, referred to RU random allocationmodule 705 for allocating, e.g. OFDMA resources (sub-channels),implements the part of the invention that regards node 600, i.e.transmitting operations for a source node, receiving operations for areceiving node.

On top of the Figure, application layer block 701 runs an applicationthat generates and receives data packets, for example data packets of avideo stream. Application layer block 701 represents all the stacklayers above MAC layer according ISO standardization.

The present invention in its different aspects is now illustrated usingvarious exemplary embodiments from the nodes' perspective (FIGS. 7, 8 b,12 and 14) and from the AP's perspective (FIG. 8a ).

Some of these exemplary embodiments (e.g. first, second and thirdexemplary embodiments) are described in the context of IEEE 80211.ax byconsidering OFDMA sub-channels. In the description of these embodiments,the term legacy refers to non-802.11ax nodes, meaning 802.11 nodes ofprevious technologies that do not support OFDMA communications.

Application of the invention is however not limited to IEEE 802.11axcontext. Furthermore, embodiments of the invention do not necessarilyrely on the usage of an OFDMA scheme, and any other scheme definingalternate resource units (e.g. TDMA sub-channels) or more generallychannels to be access simultaneously or sequentially can also be used.

FIGS. 8a and 8b illustrate, using two flowcharts, general steps of afirst exemplary embodiment of the present invention for providing randomaccess to OFDMA Resource Units for (uplink) multi-user OFDMAtransmission in the context of 802.11. FIG. 8a illustrates theembodiment from the AP's perspective and FIG. 8b illustrates theembodiment from the node's perspective.

FIG. 9 illustrates exemplary communication lines resulting from animplementation of the first exemplary embodiment of FIGS. 8a and 8b .Although the illustration shows a WLAN system using a multi-channelincluding a 40 MHz bandwidth channel having a set of 8 OFDMA resourceunits, the number of 20 MHz bands forming the overall composite channeland/or the number of OFDMA resource units per 20 MHz channel bandwidththereof may vary.

Also, the application of the invention is raised through examples thatuse the trigger frame mechanism sent by an AP for a multi-user uplinktransmissions according to TF 430 of FIG. 5a . Of course, equivalentmechanisms may be used in a centralized or in an adhoc environment (noAP).

The allocation method according embodiments of the present invention isapplicable to any 802.11 frame which is transmitted in the non-HTduplicate format (e.g. a trigger frame or any control frames which, in apreferred implementation, contains an information element informingabout the number of resource units per 20 MHz channel band). It shouldbe noted that the number of resource units available and/or theirrepartition are not necessarily signalled in the trigger or controlframe. For example, the signalling may be performed in a RU (typicallywith less than 20 MHz bandwidth) previously reserved in a downlink OFDMATXOP. The number of resource units and/or their repartition may also bepredefined or shared by a different means between the nodes of thenetwork.

The methods of FIGS. 8a and 8b are implemented by at least two stations600, at least one being a simple station 101-107 and the other an accesspoint. In other words, there are at least one source node having data totransmit to the other node, namely a destination or receiving node (likethe AP for uplink communications).

Flowchart of FIG. 8a illustrates the algorithm performed to prepare atrigger frame and transmit it onto the wireless channel. This algorithmis performed by a receiving node like preferably an access point.

Although not limited in this respect, one way to implement thisalgorithm is as follows.

Step 800:

The AP determines the number of Resource Units to consider for themulti-user TXOP upon being granted. This determination is based on theBSS configuration environment, that is to say the basic operationalwidth (namely 20 MHz, 40 MHz, 80 MHz or 160 MHz channels that includethe primary 20 MHz channel according the 802.11ac standard). For thesake of simplicity, one may consider that a fixed number of OFDMA RUs isallocated per 20 MHz band: in that case, it is sufficient to addBandwidth signalling in the TF frames (i.e. 20, 40, 80 or 160 MHz valueis added). Typically, this information is signaled in the SERVICE fieldof the DATA section of non-HT frames according the 802.11 standard.These choices are advantageous for keeping legacy compliancy for themedium access mechanism.

In a preferred implementation, the TF contains an information elementindicating that the multi-user TXOP is of random type (e.g. TF-R); thatis to say multiple stations 600 can randomly contend for access a RUinside the OFDMA TXOP. That explicitly indicates that those sourcestations 600 are allowed to execute algorithm of FIG. 8b . As anexample, this information element may take the form of one bit, whichmay be the reserved bit 7 (referenced as 350) of the QoS Control Fieldheader (300) of a 802.11 MAC frame according to FIG. 3 c.

Step 801:

AP sends a TF with an indication of the bandwidth of the intended TXOP.

It is expected that every nearby legacy node (non-802.11ax node) canreceive the TF on its primary channel. Each of these nodes then sets itsNAV to the value specified in the TF frame: the medium is thus reservedby the AP.

Flowchart of FIG. 8b illustrates the behaviour of a source station 600for determining a random allocation of an OFDMA RU in direction to thereceiving node (like an AP). Although not limited in this respect, oneway to implement this random allocation is described hereafter.

Step 810:

At step 810, the source node 600 has to verify that it has received a802.11a frame in a non-HT format. Preferably, the type of the frameindicates a trigger frame (TF), and the Receiver Address (RA) of the TFis a broadcast or group address (i.e. not a unicast addresscorresponding specifically to the source node 600 MAC address).

Upon receiving the trigger frame, the channel width occupied by the TFcontrol frame is signaled in the SERVICE field of the 802.11 data frame(The DATA field is composed of SERVICE, PSDU, tail, and pad parts). Thedetermination that the control frame is a Trigger Frame may be indicatedthrough the frame control 301, which indicates the type of the receivedframe. The frame control 301 may include in addition to the type of theframe a sub-type for either identifying the trigger frame or the type ofthe trigger frame such as a TF-R. The support of random OFDMA TXOP maybe determined also by the usage of reserved bit 7 (referenced as 350) ofthe QoS Control Field header (300) of the TF (in case of TF and randomTF have same type/sub-type identifications).

If the received TF indicates a random type (e.g. trigger frame forrandom access TF-R, or a TF designating at least one RU of random type),then the algorithm continues in step 811.

Step 811:

All Resource Units (OFDMA sub-channels) supporting the OFDMAcommunication are known at this stage (e.g., the number of OFDMA RUs isfixed per 20 MHz band, and the operating band of the composite channelis detected in 810). Also, OFDMA sub-channels supporting random access(Random RUs) are designated in the signaling of the TF according to oneimplementation of the invention. In the following available RUs (orResource Units for the OFDMA communication) will refer to Random RUsonly if otherwise stated.

One can note that a transmitting node 600 is a node having at least oneactive backoff engine 311, which means with a value greater than 0 uponthe reception of the trigger frame (because the AP preempts any mediumaccess against stations, due to a PIFS duration lesser than the DIFSduration of the stations according the 802.11 standard). More than onebackoff engine may be active per node when IEEE 802.11e EDCA involvingaccess categories is implemented and data is ready for transmission fromdifferent transmission queues. The backoff counter value of an activebackoff engine at the time the wireless medium was preempted by the APis referred to as pending backoff value. Thus a plurality of pendingbackoff values may be pending at a time for a given node.

According to embodiments of the invention, pending backoff countervalues of transmitter stations 600 are advantageously reused forrandomly determining (selecting) a RU onto the OFDMA TXOP fortransmitting data and/or for determining the eligibility of a station totransmit data.

First, the station determines according to the first exemplaryembodiment if at least one of its pending backoff values fits in thenumber of Resource Units for the current OFDMA communication (i.e.Random RUs). As an example, if the number of random RUs is 8 (asillustrated in FIG. 9 for an exemplary 40 MHz band, wherein each 20 MHzchannel band contains 4 OFDMA Resource Units), then all stations 600having at least one active backoff engine with a pending backoff valuethat is less than or equal to 8 are considered as eligible for havingaccess onto a Resource Unit.

More than one pending backoff value may fit into the number of ResourceUnits at a station leading to a plurality of transmission queueseligible for serving data to be transmitted in a RU. The virtualcollision handler 312 may handle a number of eligible queues (typicallyup to 4) in relation with the transmission capabilities of each station600: that is to say according to the maximum number of concurrent OFDMAtransmissions supported by station 600.

If the capabilities of the node allow for only one channel of thecommunication medium to be used per node (e.g. maximum number ofconcurrent OFDMA transmissions per node is equal to one), thus in caseof a plurality of pending backoff values of the node fitting into thenumber of Resource Units (i.e. a plurality of transmission queues areeligible), the virtual collision handler 312 takes the responsibility ofresolving the selection of a single transmission queue. The selectionmay be based on EDCA traffic class prioritization. In a variant, theselection may be based on the corresponding pending backoff values; thetransmission queue associated with the lowest pending backoff value isselected. Indeed, this corresponds to the transmission queue that wouldhave accessed first the communication medium if a conventional 802.11contention-based access mechanism were implemented (i.e. when thebackoff counter reaches zero).

If the capabilities of the node allow for more than one channel to beused per node (e.g. maximum number of concurrent OFDMA transmissions pernode greater than one), thus in case of a plurality of pending backoffvalues of the node fitting into the number of Resource Units, thevirtual collision handler 312 takes the responsibility of selecting aplurality of eligible transmission queues to contend for access overavailable RUs. The number of eligible transmission queues selected bythe virtual collision handler 312 is kept however smaller than or equalto the maximum number of possible concurrent channels. If the number ofeligible transmission queues is greater than the maximum number ofpossible concurrent channels, the selection may be based on EDCA trafficclass prioritization. For example, a transmitting station supportingonly two concurrent OFDMA transmissions and having three active backoffengines (for example, a pending backoff value of 3 for VOICE trafficqueue, a pending backoff of 4 for BACKGROUND traffic queue, and apending backoff of 5 for VIDEO traffic queue) would select VOICE andVIDEO traffic queues to serve data for transmission on two RUs of thecommunication medium. In a variant, the selection may be based on thecorresponding pending backoff values; lower pending backoff values aregiven higher priority and are selected first. By reusing the aboveexample, VOICE and BACKGROUND traffic queues would be selected in thisvariant.

Eligible transmission queues that were not selected (if any) areconsidered as collided and a new backoff counter value is redrawn.

FIG. 14 illustrates, using a flowchart, a possible implementation of thevirtual collision handler.

At step 1401 the maximum number of concurrent OFDMA transmissionssupported by the node (MAX_NB_TX) is obtained. This information may beobtained from memory of the node and may correspond to a design orconfiguration option of the communication node.

At step 1402 the number of eligible backoff engines (MAX_BK_ENG) isdetermined. This corresponds to active backoff engines associated withtransmission queues within the node that are eligible according to agiven criteria to serve data if the node contents for access to a RU.

At step 1403 a test is performed to check if the number of eligiblebackoff engines (MAX_BK_ENG) is greater than the maximum number ofconcurrent OFDMA transmissions supported by the node (MAX_NB_TX). If thetest is positive, a number of backoff engines equal to (or lower than)the number of concurrent OFDMA transmissions MAX_NB_TX is selected(1404); otherwise all the backoff engines may be selected (1405). Theselection at step 1404 may be based on the priority of the traffic or onthe pending backoff values.

At step 1406, transmission queues associated with selected backoffengines are used for data transmission.

If pending backoff values of two or more active backoff enginescorresponding to eligible queues have the same value for the station600, only one eligible queue among the virtually-collided queues may beselected. The other(s) virtually-collided queues are not selected.Alternatively, if two or more active backoff engines corresponding toeligible queues have the same value and the station is capable ofsupporting at least two concurrent OFDMA transmissions, then at leasttwo virtually-collided queues may still be selected. In this variant, aone-to-one mapping between the pending backoff values and the RUs maynot be applied and a selection method of the RUs over which transmissionis performed that is independent from the pending backoff values may beused.

In step 811, a station 600 is determined as an eligible station if itcontains at least one eligible transmission queue (as illustratedbefore, a transmission queue being stated as eligible if itscorresponding pending backoff value is not greater than the number ofOFDMA sub-channels for example, or any threshold value). As a result, aneligible station 600 may contain both eligible and non-eligible queues.In contrary, a transmitter station 600 that is stated as non-eligiblestation contains only non-eligible transmission queues (in this case,all pending backoff values are greater than the number of OFDMAsub-channels specified through the received TF).

According to embodiments of the invention, the RUs that can be randomlyaccessed are indexed (e.g. sequentially numbered). The indexing iseither predefined between the stations of the network, or signalled bythe AP in the trigger frame or in a separate control frame. Thesignalling by the AP may be advantageous if the number of RUs may vary.The indexing may concern only RUs available for random access or all theRUs. If only available RUs are indexed, the mapping between the backoffcounter values of the stations and the RUs may translate more easily.

Second, and according to one implementation, if the pending backoffvalue is not greater than the number of OFDMA sub-channels, the stationis allowed to access to the RU indexed by the selected backoff value.Typically, a transmitting station having a pending backoff value of 3 isallowed to send data onto the RU having index 3 (in the context of FIG.9 for an exemplary OFDMA TXOP of 8 RUs).

As the computation of initial backoff value follows a random procedureaccording the 802.11 EDCA, then benefit is taken from such randomizationfor selecting an OFDMA Resource Unit. It is a clear advantage to keeprelying on classical random generation resources already present andimplemented in commonly known 802.11 hardware. A further advantage is toavoid implementing an additional step of drawing a random number forallocating a RU.

In step 811, the stations 600 which have determined that they are noteligible stations (e.g. each of their pending backoff values is greaterthan the number of available RUs) will conduct the following step beforefinishing the algorithm: each non-eligible transmitter station 600 maydecrement each of its active backoff value(s) of a value correspondingto the number of RUs inside OFDMA TXOP.

More generally, backoff engines 311 associated with non-eligibletransmission queues either belonging to a non-eligible station or to aneligible station, may update their pending backoff value, for example bysubtracting the number of available RUs. Alternatively, pending backoffvalues are updated by subtracting a value, typically lower than thenumber of available RUs. In a further variant, the pending backoffvalues are not updated.

Backoff engines 311 associated with eligible transmission queues andselected for serving data for transmission may reset their backoffcounter (drawing of new backoff value) if remaining data is stillavailable in the transmission buffer.

Backoff engines 311 associated with eligible transmission queues and notselected for serving data for transmission (collided queues or virtuallycollided queues) may reset their backoff counter.

If no more data is available for transmission, the corresponding backoffengine is switched to inactive and its backoff value is not taken intoaccount.

Update of the backoff values should occur any time before thetransmission opportunity (TXOP) of typically an uplink multiusertransmission ends so that conventional communication will resume usingupdated backoff values.

Step 812:

The station 600 shall respond to a TF with at least one 802.11 PPDUframe (PPDU means PLOP Protocol Data Unit, with PLOP for Physical LayerConvergence Procedure; basically a PPDU refers to an 802.11 physicalframe) in an 802.11ax format after a SIFS period in the so-determined atleast one Resource Unit of the OFDMA TXOP.

If the transmitting node estimates that, at the end of the OFDMA RUtransmission, there will still have the buffered data to be sent to thedestination, then the transmitting node 600 indicates in at least one ofthe sent MPDUs (typically the last one) this information of having moredata to send (typically by setting the “more data field” bit in theFrame Control header (301) of 802.11 MAC frame).

Back to flowchart of FIG. 8a , the TXOP can be terminated by thedestination node (AP).

Step 802:

The destination node (like the AP) will send an acknowledgment relatedto the received MPDUs from multiple users inside the OFDMA TXOP.Preferably, the ACK frame is transmitted in a non-HT duplicate format ineach 20 MHz channel covered by the initial TF's reservation. Thisacknowledgment is necessary for the multiple source nodes 600 todetermine if the destination (AP) has well received the OFDMA MPDUs, asthe source nodes are not able to detect collisions inside their selectedRUs. The source nodes 600 may flush the buffered data that weresuccessfully transmitted (step 813 of flowchart 8 b).

The Step 803 is relative to additional embodiments of the invention, andwill be described further in regards to FIG. 10.

FIG. 9 illustrates exemplary communication lines resulting from animplementation of the first exemplary embodiment of the invention. An APsends a trigger frame for random access on an overall exemplary 40 MHzoperational band (meaning the TF 430 is duplicated on two 20 MHzchannels). This example suggests that the network is configured tohandle 4 OFDMA Resource Units per each 20 MHz channel (all stations areaware of this configuration). A total of 8 RUs are thus available overthe two 20 MHz channels.

It is assumed in the illustration of FIG. 9 that all the RUs areavailable for random access and that they are indexed from 1 to 8 (RU#1to RU#8). Reference 410-2 corresponds to RU#2 and reference 410-4corresponds to RU#4.

The area 900 indicates the timing of execution of random RU selectionaccording to embodiments of the invention. This corresponds to the steps810 to 812 of flowchart 8 b. Reference 910 indicates the pending backoffvalue of each transmitter station before the wireless medium waspreempted by the AP.

Each station STA1 to STAn is assumed to be a transmitter station withregards to receiving AP, and as a consequence, each station have atleast one active backoff engine. For the sake of clear illustration,only one backoff value per station is represented in the figure, but thecase of several EDCA backoff values per station can also be supported byembodiments of the invention as stated previously.

The station STA1, executing the algorithm of flowchart 8 b, will notconsider itself as an eligible station for sending data on a RU ofcurrent TXOP because it has a backoff value of 10 which is greater thanthe maximum number of OFDMA RUs (equals 8).

The station STA2, executing the algorithm of flowchart 8 b, willconsider itself as an eligible station for sending data on a RU ofcurrent TXOP because it has a backoff value of 3 which is lesser thanthe maximum number of OFDMA RUs. It will send an OFDMA PPDU 230 insidethe RU indexed by the backoff value, so the number 3.

The stations STA3 and STA_(n-1), executing the algorithm of flowchart 8b, will consider themselves as eligible stations for sending data on aRU of current TXOP because each one has a backoff value of 4 which isnot greater than the maximum number of RUs. Both will send an OFDMA PPDU230 inside the RU indexed by the backoff value, so the number 4. One maynote that two stations will use the same Resource Unit (410-4), thusconducting to a collision. The ACK 240, sent by the AP, is the onlymeans for those stations to be alerted about such event.

One may also note that some Resource Units will not be used (for exampleRU indexed 2 (410-2), 5, 7 and 8). This is due to the randomizationprocess. In dense environments like 802.11ax standard considers, therewould be limited under-usage of RU as there is more chance to havecompeting stations having a same backoff expiration occurrence.

FIG. 13 is an exemplary illustration similar to FIG. 9. In FIG. 13,pending backoff values at the time the trigger frame (of type TF-R) wastransmitted and the resulting (updated) backoff values after accessingthe RUs are represented. Note that RUs #1 to #4 are sub-divisions of theprimary 20 MHz channel and that RUs #5 to #8 are sub-divisions of asecondary 20 MHz channel.

According to first exemplary embodiment of the invention, the multiplestations (STA₁ to STA_(n)) use their pending backoff counts as criteriafor accessing the UL MU TXOP, and particularly to select a Resource Unit(RU) to be used for the next UL MU TXOP. Due to the random nature of thebackoff values, collision may still possibly occur over some RUs, whileother RUs may remain free. The scenario of the figure is chosen toillustrate these possibilities.

Upon receiving a TF-R, the stations analyze if they are eligible toaccess a RU:

-   -   the current (pending) backoff count is tested if it fits in the        number of available RUs (for example, backoff value <Nb of        random RU defined by the TF-R); and    -   an eligible station may transmit inside the RU corresponding to        its backoff value (for example, RU index=backoff value) and        redraws its backoff value after that if further data needs to be        transmitted.

In one implementation, at the UL MU TXOP end, all other (non-eligible)802.11ax stations should have updated their backoff counts (e.g.decrease their backoff counts by the number of available RUs specifiedin TF-R; for instance, for station STA1, pending backoff value 10 isreduced by 8 which is the number of available RUs in this example).

The proposed solution is fully distributed and is advantageous sincethere is no need to perform a specific random computation for TF-R(backoff is already randomly generated), it is 802.11 standard compliant(EDCA priority is kept), and collisions occurring over RUs are limitedto 802.11ax stations (implementing the embodiments of the invention) andnot involving legacy stations. Also, the proposed station-eligibilityprocedure reduces the probability of RU collisions.

FIG. 10 presents the format of a ‘RU Information Element’ (1010), whichmay be used in the first and second exemplary embodiments of the presentinvention.

The ‘RU Information Element’ (1010) may be used by the AP to embedadditional information inside the random trigger frame (TF-R) related tothe OFDMA TXOP. Its format follows the ‘Vendor Specific informationelement’ format as defined in IEEE 802.11-2007 standard. The ‘RUInformation Element’ (RU IE, 1010) is a container of one or several RUattributes (1020), having each a dedicated attribute ID foridentification. The header of RU IE can be standardized (and thus easilyidentified by stations) through the Element ID, OUI, OUI Type values.

The RU attributes 1020 are defined to have a common general formatconsisting of a one-byte RU Attribute ID field, a two-byte Length fieldand variable length attribute specific information fields.

The usage of Information Element inside the MAC frame payload is givenfor illustration only, any other format may be supported.

As a variant, the RU attribute body (further referenced by comprisingelements from 1021 to 1023 according to FIGS. 10 and 11) may be directlylocated in the frame body field 302 of the trigger frame.

The choice of embedding additional information in the MAC payload isadvantageous for keeping legacy compliancy with the medium accessmechanism, because any modification performed inside the PHY header ofthe 802.11 frame would have inhibited any successful decoding of the MACheader (the Duration field would not have been decode, so the NAV wouldnot have been set by legacy devices).

As a result, the trigger frame is advantageously compatible with varioustransmission widths according 802.11 standard. As an example, thetransmission width is a multiple of legacy 20 MHz width (this is thecase when the TF follows the 802.11a non-HT duplicated format accordingto FIG. 5a ; in another exemplary case, the TF may alternatively use abandwidth formed of at least one 20 MHz channel in a TXOP granted by atleast one 802.11a control frame). As another example, the TF may betransmitted with a lower width (less than 20 MHz): as example, the APcan transmit the TF inside a RU sub-channel of a downlink OFDMA TXOP.

Although not limited in this respect, one way to implement the random RUallocation is described hereafter based on said IE formats.

In a first implementation, the trigger frame contains a list ofavailable RUs for random access.

To do so, the TF contains a specific information element 1010 in theframe body 302 of the 802.11 MAC frame, which contains the RU attribute1020A according to FIG. 10.

As shown in the Figure, a dedicated RU attribute for the ‘RU List Info’attribute (1020A) follows the following format:

-   -   The Attribute ID is a dedicated value identifying the ‘RU List        Info’. A value unused in the standard, e.g. in the range 19-221,        may be selected. This one-byte value is a tag starting the ‘RU        List Info’.    -   RU nb (1021) gives the number of Resource Units which are        available for usage. This number also gives the number of        entries in the next field.    -   RU list (1022), is the list of RU indexes supporting the random        allocation for the current OFDMA TXOP.

A variant (not shown in the figure) may consist in describing thechannel specificities of each RU, that is to say the frequenciesbounding each (available) OFDMA Resource Unit. This is in case the OFDMAgranularity map is not known by a source station joining the BSS ownedby the AP.

Thus, the behavior of step 811 performed by source station(s) 600 may beimplementation as follows for performing the random RU allocation:

-   -   an eligible station is determined through a counter backoff less        that the number of available RUs (given by RU_nb 1021);    -   the RU slot is selected through its index in the list RU_list        1022.

In a second implementation, based on the first implementation, thetrigger frame also contains a list of scheduled RUs in addition of thosefor random access.

FIG. 11 presents this other format (1020B) for the ‘RU Access Info’attribute. An attribute ID may be assigned to that format and may bedifferent from the 1020A's one.

A list of scheduled RU (1023) is provided by the AP in the triggerframe, wherein each scheduled RU is allocated to a given station(already registered to the AP) identified by its ‘AssociationIdentifier’ (or ‘AID’ according 802.11 standard).

This implementation is advantageous for mixing scheduled and randomcommunications from multiple source stations, which are triggered by asingle TF.

A variant (not shown in the figure) may consist in describing a list ofRU (similar to 1023) comprising all the RUs (that is to say both randomand scheduled types), wherein a RU with an associated AID set to 0indicates that this RU is of random type. In that case, 1021 and 1022may not be required.

The random RU allocation procedure concerning the behavior of step 811described here above for the first implementation is still applicable.

This second implementation is advantageous when a source stationindicates that it has still data to send as explained in step 812: theAP may react to that notification through the step 803, wherein itprepares the next OFDMA TXOP's profile: it may schedule for that sourcestation 600 a fixed RU slot inside the scheduled list 1023 (instead ofletting the source fighting to randomly obtain a random Resource Units).

In a further implementation of step 803, the AP may adjust the number ofrandom RUs compared to the scheduled RUs for next attempts inconsideration of various elements, which may be (without limitation):

-   -   the number of registered/detected stations inside the wireless        cell;    -   the detected collisions seen by the AP: for example, the number        of collided random RUs in past OFDMA TXOPs;    -   the smooth increase/decrease of unicast communications in        direction to the AP.

As one can note, the various alternative embodiments presented in FIGS.10 and 11 are compatible one with each other, and may be combined totake advantage of their respective advantages.

As a variant of the random allocation procedure, the total number of RUscomposing the OFDMA TXOP are all still considered for selection based onbackoff value (even if some RU slots are indicated as reserved through1023 for scheduled RUs). Pending backoff values are thus mapped to allRUs and not only to RUs available for random access.

Thus, in a variant implementation of step 811, an eligible station stillneeds to have its backoff count lesser than the total number of RUs,however if the backoff count is lower than the total number of RUs, butpoints to a position of a scheduled RU from list 1023, then the eligiblestation defers it access (as if there is a virtual collision).

In a second variant, said eligible station which falls into an occupiedRU slot may select the nearest (as example+1 or −1) available RU fromrandom list 1021.

A second exemplary embodiment is based on the first exemplary embodimentwith all its variants wherein the eligibility is determined by comparingthe pending backoff values to a threshold value instead of the number ofavailable RUs.

In this second embodiment, backoff engines 311 associated withnon-eligible transmission queues (either belonging to a non-eligiblestation or to an eligible station), may update their pending backoffvalue by subtracting the threshold value. Alternatively, the pendingbackoff is not updated.

A third exemplary embodiment is based on the first or second exemplaryembodiments with all their variants wherein the behavior of step 811performed by source station(s) 600 is modified such that the eligiblestations are not limited to those having a pending backoff value lesserthan the available RU slots. Thus, all source stations having pendingdata in direction to the receiving station (e.g. the AP) can randomlyselect a RU slot: in the present case, the backoff value for determiningthe RU index (RU_index) must fit into the number of available RUs(1021).

So, if backoff value is greater than the number of random RU slots, theRU_index may be the remainder of the Euclidean division of backoff valueby the maximum available RU slots:RU_index=backoff_value mod RU_nb

wherein ‘mod’ is the modulo operation.

More generally, any mapping between the backoff_value and the RU_indexcan be applied. Increasing the number of eligible stations reduces theprobably to have empty RUs but increases the risk of collisions.Choosing the number of available RUs as a threshold for deciding abouteligibility of stations is one possible implementation, but thisthreshold can be increased or reduced to define a tradeoff betweenunused RUs and collisions for better bandwidth usage efficiency.

A fourth exemplary embodiment is based on the first or second exemplaryembodiments with all their variants wherein the behavior of step 811performed by source station(s) 600 is modified such that the index ofthe RU over which the access is performed is determined by a process notdependent on the pending backoff values (e.g. an additional randomprocess). In this embodiment, advantage is brought by applying theeligibility criteria to the transmission queues for reducing the numberof candidates for contention and hence collisions as discussed above.

More generally, flowchart of FIG. 12 illustrates the behaviour of asource node 600 for contending access to a communication channelaccording to embodiments of the invention.

The source node implements a medium access mechanism with collisionavoidance based on a computation of backoff values. A backoff countervalue corresponds in one of its aspects to a number of time-slots thenode waits, after the communication medium has been detected to be idle,before accessing the medium. Other aspects of the backoff counter usageaccording to embodiments of the invention are detailed herebelow.

The communication channel may be any communication resource among aplurality of resources of the wireless network shared in time and/orfrequency over which nodes may contend for access. The communicationchannel may be a resource unit (sub-channel), a 20 MHz channel or anypart of the composite channel. The channels may be multiplexed infrequency (OFDMA), in time (TDMA) or in any combination of time andfrequency.

At 1201, at least one backoff counter value computed by the node isobtained. At 1202, identifiers of the plurality of communicationchannels are obtained. The identifiers may be indexes obtained out of anindexing process of the communication channels. The identifiers may alsobe sequence numbers associated with the communication channels.Preferably, only channels that are available for random access areindexed. This allows to make an easy one-to-one mapping for examplebetween a backoff counter value and the identifier (index) of onechannel (cf. step 1204). The identifiers may be generated locallyaccording to a process known to the nodes of the wireless network orcommunicated by the AP.

Optionally, the obtained backoff value is used to determine if the nodeis eligible to contend access to a communication channel. Typically, abackoff value which has no corresponding channel identifier to map intomay cause the node which computed that backoff value to be considered asnot eligible.

In a simple implementation, the computed backoff value should be notgreater than the highest index of the communication in order for thenode to be eligible.

In a variant where the selection step 1204 is not based on a one-to-onemapping for example between a backoff counter value and the identifier(index) of one channel, a different eligibility determination may beperformed: for example, a reference backoff value (threshold value) mayhave been specified by the node granting the medium access (for exampleembedded in the trigger frame 430), and this reference is considered bysource node 600 as a maximum threshold for its local backoff values inorder to determine itself as an eligible node.

If the node is tested as eligible, the method continues with the steps1204 and 1205. Otherwise, the flowchart is re-executed when a newbackoff counter value is computed.

If the step 1203 is not implemented, the steps 1204 and 1205 areexecuted after the step 1202. This is for example the case when all thenodes are by default eligible thanks to the implementation of a mappingbetween all possible values of the backoff counter and the identifiersof the available communication channels. This mapping may be implementedby a modulo function as described above.

At 1204, a communication channel is selected among the plurality ofcommunication channels based on the obtained backoff counter value. Inone implementation where each channel is associated with a sequencenumber, the selected channel has an associated sequence number equal tothe obtained backoff counter. In an alternate embodiment, acommunication channel is selected among the plurality of communicationchannels using a process not dependent on the backoff value used fordeciding about the eligibility of the node, such as a random processrelying on the drawing of independent random values or a deterministicprocess.

At 1205, the node contends access on the selected channel by sending adata frame on that channel.

Although the present invention has been described hereinabove withreference to specific embodiments, the present invention is not limitedto the specific embodiments, and modifications will be apparent to askilled person in the art which lie within the scope of the presentinvention.

Many further modifications and variations will suggest themselves tothose versed in the art upon making reference to the foregoingillustrative embodiments, which are given by way of example only andwhich are not intended to limit the scope of the invention, that beingdetermined solely by the appended claims. In particular the differentfeatures from different embodiments may be interchanged, whereappropriate.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that different features are recited in mutuallydifferent dependent claims does not indicate that a combination of thesefeatures cannot be advantageously used.

The invention claimed is:
 1. A wireless communication method in awireless network comprising an access point and a plurality of nodes,the method comprising the following steps, at one of the nodes:receiving a trigger frame from the access point, the trigger framereserving a transmission opportunity on a 20 MHz channel, the reserved20 MHz channel being split into OFDMA resource units in the frequencydomain including one or more random OFDMA resource units the nodes canaccess using a contention scheme; obtaining a random backoff value thatthe node initializes; comparing the backoff value to a threshold valuewhich is based on a number of the one or more random OFDMA resourceunits; determining if the node is eligible to contend for access on theone or more random OFDMA resource units based on the result of thecomparing; and transmitting data on one or more of the one or morerandom OFDMA resource units if the node is determined to be eligible. 2.The wireless communication method of claim 1, wherein the node iseligible to contend for access if the random backoff value is notgreater than the threshold value.
 3. The wireless communication methodof claim 1, wherein the threshold value is received from the accesspoint.
 4. The wireless communication method of claim 3, wherein thethreshold value is included in the trigger frame.
 5. The wirelesscommunication method of claim 1, wherein the node comprises a pluralityof transmission queues for serving data at different priorities.
 6. Thewireless communication method of claim 5, wherein the steps of obtaininga random backoff value and comparing the backoff value to a thresholdvalue are executed for each transmission queue, and wherein a queue iseligible to serve data depending on the result of comparing the randombackoff value associated with the queue with the threshold value.
 7. Thewireless communication method of claim 6, wherein a queue is eligible toserve data if the random backoff value associated with the queue is notgreater than the threshold value.
 8. The wireless communication methodof claim 6, wherein the node is eligible to contend for access if atleast one of its queues is eligible to serve data.
 9. The wirelesscommunication method of claim 6, wherein if a plurality of queues areeligible to serve data, one queue is selected among the plurality ofeligible queues for serving data to be transmitted on the random OFDMAresource unit in the transmitting step.
 10. The wireless communicationmethod of claim 6, wherein if a plurality of queues are eligible toserve data and the node is capable of transmitting data over a pluralityof transmission opportunities on one or more random OFDMA resourceunits, a plurality of eligible queues are selected for serving data tobe transmitted on one or more random OFDMA resource units in thetransmitting step.
 11. The wireless communication method of claim 9,wherein the selection of one eligible queue is based on the trafficpriority associated with the queue or queues or on the random backoffvalue(s) obtained for the queue or queues.
 12. The wirelesscommunication method of claim 1, wherein the random OFDMA resource uniton which data is transmitted is selected among a plurality of randomOFDMA resource units the nodes can access using a contention schemebased on the obtained random backoff value.
 13. The wirelesscommunication method of claim 1, wherein the random OFDMA resource unitis a resource unit resulting from the division of a 20 MHz channel intoOFDMA sub-channels.
 14. The wireless communication method of claim 1,wherein the trigger frame is received from the access point and whereinthe data is transmitted at the transmitting step to the access point.15. The wireless communication method of claim 1, wherein the randombackoff value is the number of 802.11 time-slots the node must wait whena medium is sensed idle before the node grants itself a transmissionopportunity (TXOP).
 16. The wireless communication method of claim 1,further comprising a step of updating a random backoff value bysubtracting the number of random OFDMA resource units to be used for anext data transmission.
 17. A communication device in a wireless networkcomprising an access point and a plurality of nodes, the communicationdevice being one of the nodes and comprising: means for receiving atrigger frame from the access point, the trigger frame reserving atransmission opportunity on a 20 MHz channel, the reserved 20 MHzchannel being split into OFDMA resource units in the frequency domainincluding one or more random OFDMA resource units the nodes can accessusing a contention scheme; means for obtaining a random backoff valuethat the node initializes; means for comparing the backoff value to athreshold value which is based on a number of the one or more randomOFDMA resource units; means for determining if the node is eligible tocontend for access on the one or more random OFDMA resource units basedon the result of the comparing; and means for transmitting data on oneor more of the one or more random OFDMA resource units if the node isdetermined to be eligible.
 18. A wireless communication system having anaccess point and at least one node according to claim
 17. 19. Anon-transitory computer-readable medium storing a program which, whenexecuted by a microprocessor or computer system in a device of awireless network, causes the device to perform the method of claim 1.